The electronic stabilization effect of the substituents on the nitrosyl-containing molecules has been studied by computing the electronic energy change (DeltaE) in the isodesmic reactions, R-NO + H-2 --> H-NO + RH, at the B3LYP/6-311 + G(d,p) level of theory. A total of 15 substituent groups (R) which span a wide range of sigma and pi donating/withdrawing strength have been included. Studies were extended to three isovalent analogues, namely, the NS-, PO-, and PS-containing molecules, with the same group of substituents. Computational results show that sigma-withdrawing groups strongly destabilize the NS molecules, whereas in striking contrast, they substantially stabilize the PO molecules. The a effect is much less pronounced in either the NO- or the PS-containing series. The pi effect, however, is more unidirectional. Specifically, the pi-donating groups contribute positively to the molecular stability in all four isovalent analogues. A semiquantitative analysis, which is based on the frontier molecular orbital theory, is presented. The vastly different responses of the isovalent groups toward the various substituents in their thermal stabilities, as well as their structures, are also discussed.